Ball grid array packages (BGA's) are known in the electronics industry to provide the attachment of a high density of input and output (I/O) solder pads on a chip carrier where a chip is already mounted on a carrier. BGA is a type of Chip Scale Package structure which uses a solder ball attach process on the bottom side of the carrier to mount the chip carrier onto a substrate. The substrate could be a printed circuit board or another type of mother boards.
Bumping these BGA carriers to form the solder pads for eventual connection with the substrate requires accurate and co-planar attachment of hundreds, and often thousands of solder spheres or balls per array on the carder. For mass production, the co-planarity quality of solder balls on the carrier after the ball attach process is an important concern of the quality process. A uniform height distribution of solder balls for a co-planar attachment is necessary to provide good electrical and mechanical connections between the carrier and the substrate.
Referring to FIG. 1, one source of the non-planarity 140 problem of a ball grid array (BGA) Chip Scale package is represented where the solder balls are not uniform in height and therefore may not make good connections when they are sandwiched between a substrate 40 and the carrier 20. Hence, to optimize the co-planarity of the solder ball placement process, pre-formed solder balls are necessary. Even though pre-formed solder balls may cost more than other processes, such as screen printing with a solder paste, pre-formed solder balls which provides a constant solder ball height are preferred when co-planarity cannot otherwise be achieved.
A semiconductor device 10, such as an integrated (IC) chip, is shown mounted on a carrier 20 as a chip carrier for eventual mounting to a substrate 40. An example of a substrate 40 is a printed circuit (PC) board, a multi-chip module, or a mother board. A plurality of hemispherical cavities or flat contact pads on top of the substrate are normally arranged in a square or other geometric pattern suitable to receive the corresponding BGA solder-bumped chip carrier 20, or the like. Each of the cavities or contact pads is metalized so as to become solderable and electrically conductive to provide the electrical interconnection between the chip carrier 20 and the substrate 40. The solder-bumped chip carrier (or other member) 20 includes a plurality of BGA solder bumps 14 formed on the underside, or the bottom surface 16, of the carrier 20.
A robot is commonly used for precise solder ball placement of the pre-formed solder balls. Often, an all-in-one robot station handles the fluxing duties as well as the co-planar placement of the balls. A highly accurate robot arm equipped with a precision tooled vacuum head picks, fluxes, and places these tiny balls. The robot arm employed is often vision equipped such as the Seiko XM robot arm. Vision inspection of the solder ball loading and unloading for carrier placement avoids or traps obvious solder ball placement carrier rejects. A complete system called the MMS SA-150 BGA Sphere Attach System is available from the Motorola Manufacturing Systems of Motorola.
A robotic arm in a conventional solder ball loading system has a vacuum head with a vacuum tool tip called an end-effector. As seen in FIG. 2, a cross-section of a conventional vacuum tool tip of the vacuum head is represented. The vacuum tool tip end-effector is lowered to simultaneously pick-up all the pre-formed balls from a bulk bin containing thousands of solder spheres. However, picking the balls out of the bin with the vacuum head is not simple because the supply level of the balls is constantly changing and the balls can easily clump together at one of the vacuum head openings. Moreover, the downward force of the vacuum head can easily damage some of the solder balls contained in the bin and also jam one or more balls into the head openings.
The conventional vacuum hole is spherical and has a diameter that is larger than the spherical solder ball to enable the spherical periphery of the vacuum head opening to grip around the solder ball. However, as the BGA ball size decreases to micro sizes, such as around a diameter of 12 mils (0.305 mm), with the advance of microelectronics, the resulting smaller spherical portion of the vacuum hole becomes more difficult to make uniform.
Furthermore, as smaller solder balls decrease in size, they also increase their sensitivity to static electricity and decrease their sensitivity to gravity. These difficulties of handling 0.305 mm pre-formed solder balls result in an increasing amount of ball clusters occurring around the vacuum holes. Prior-art loading systems have thus used a vibrating or a heating device, such as a shaking-type container, to help remove these unwanted clusters of excess solder balls but still is barely effective for the gravity-resistant 0.305 mm solder balls. The success rate of picking-up a preformed 0.305 mm solder ball undamaged for making a proper connection in a conventional shaking-type container is nearly zero.
This inability to provide a clusterless and damage-proof solder ball loading process has frustrated attempts to provide BGA mounting techniques that are reliable in a mass production environment utilizing minimum in-line automation equipment, repeatable automatic process control, and high through-put productions of chip carriers on substrates such as printed circuit (PC) boards, multi-chip modules, or mother-boards. Accordingly, a need exists in the art to provide a solder ball loading system in a manner that provides co-planarity and consistency in solder ball attachment while avoiding the difficulties presented by prior hard-to-manufacture vacuum spherical holes and eliminating the extra cost and necessity of a cluster removal device at the robotic arm station.